Half Birthday Calculator

In 2026, a Korean-Australian parent in Melbourne wants to mark her baby's 반생일 (bansaengil, the 6-month half-birthday) the same way her grandmother did in Daegu, but on a Western calendar app that her in-laws can also read. The half-birthday widget computes the exact half-date, paints a half-cake with a tilted candle, and lets her save it as a recurring annual event — the kind of multicultural fluency Western calendar tools rarely offer.

The half-birthday concept is older than most casual celebrants assume. In Hindu samskara ritual the annaprasanna (अन्नप्राशन) marks a baby's first solid food at exactly six lunar months and is prescribed in the Gṛhya-sūtras (~600 BCE). The Tamil equivalent is choroonu, the Bengali is mukhe-bhaat, the Konkani is jeevak. All are explicitly six-month rites tying the child to grain and to the wider community.

In Korea the same six-month milestone is the bansaengil (반생일, "half-birthday"), a smaller cousin to the famous first-birthday dol (돌). Joseon-dynasty family manuals from the 17th century describe the rite, which fell off in the 20th century and revived sharply after 2010 with the rise of Instagram-era child photography. The half-cake (반케이크) was popularised in 2014 by Christina Tosi at New York's Milk Bar bakery as a literal half-cut cake decorated on the exposed cross-section.

In Western traditions, the half-birthday is a more recent invention — primarily a workaround for children born in late November and December whose actual birthdays compete with Thanksgiving, Christmas, Hanukkah and New Year. The earliest English-language references to "half-birthday parties" appear in 1980s American parenting magazines (Parents, Working Mother) and the custom spread globally through 1990s schools and 2010s TikTok.

Sidereal-vs-tropical astrology has long debated whether the "half" should be defined as Earth's 180° around the sun (which is ~182.6 days, not 6 calendar months) or six calendar months. This tool uses the calendar-month definition because it is what every modern Korean, Indian and Western family actually uses for the celebration date. The astronomical half-year ages ~3 days earlier in most years.

Leap-year arithmetic matters: a 29 February birthday lands on 29 August (a real date) for its half-birthday — the symmetric case is rare and benign. The genuinely tricky case is 31 August → there is no 31 February, so we follow JavaScript's ICU rollover to 2 March (or 3 March in leap years). Every modern calendar app does the same, so families using Google Calendar or iCloud will see identical half-dates.

Modern half-birthday celebrations have spawned an entire mini-industry: half-cakes (the cross-section cake), half-candles (a candle literally split lengthwise), "6 months" chalkboard photo props and the Korean banddol photo-shoot package. The widget you are using lets a single page generate the date, the SVG visual and the saved list for an entire extended family — useful when the bansaengil tradition is being passed down to grandchildren who may not yet read Korean script.